Semipermeable membranes prepared from various polymers can be utilized to separate components of solutions by various techniques such as reverse osmosis, electrodialysis or ultrafiltration. For instance, U.S. Pat. Nos. 3,679,538 and 3,801,404. disclose open celled microporous films of crystalline polymers such as polypropylene, and methods for their preparation. U.S. Pat. No. 4,240,914 discloses selective permeable anisotropic membranes of polyimide polymers having a dense skin layer on one surface of a porous layer.
Semipermeable membranes of polybenzimidazole polymers have been found useful for reverse osmosis processes. For example, U.S. Pat. No. 3,720,607 discloses semipermeable polybenzimidazole membranes having a thin layer of higher density on one surface. U.S. Pat. Nos. 3,737,042; 3,841,492; and 3,851,025 disclose similar membranes, in forms including flat films and hollow fibers, which have been annealed to improve separation properties of the thin surface portions of the membranes which perform the separation function. Unfortunately, the annealing treatments which improve the dense semipermeable layers do not always improve the porous layers which serve a support function. For example, U.S. Pat. No. 4,448,687 discloses mechanical problems resulting from annealing of polybenzimidazole polymer membranes. Most of these membranes must be kept moist during processing and storage.
Various composite membranes have been disclosed in the art. For example, U.S. Pat. No. 4,337,154 discloses composite semipermeable membranes comprising a porous support and a cross-linked thin film formed thereon. U.S. Pat. No. 4,230,463 discloses multicomponent membranes for gas separations which comprise a coating in contact with a porous separation membrane, in which the membrane's separation properties are largely determined by the porous separation membrane as opposed to the coating. Organic polymers which can be used for both the porous and coating portions include polysulfones and polyolefins, e.g., polypropylene. Polymers of polybenzimidazoles are not mentioned.
U.S. Pat. No. 4,378,400 discloses gas separating materials comprising a dense film of an aromatic imide polymer, optionally laminated on a porous substrate. U.S. Pat. No. 3,951,81 discloses composite semipermeable membranes comprising ultrathin cross-linked polyamide films formed either separately or in situ on porous substrates of polymers such as polysulfones. U.S. Pat. No. 4,260,652 discloses the preparation of permselective composite membranes by coating a microporous substrate with an aqueous amine solution of an imidazolone ring-containing aromatic polymer. The microporous substrate can be any known in the art, including polysulfones. Although Column 7 of U.S. Pat. No. 4,260,652 states that certain groups within the substituents listed for the imidazolone ring-containing aromatic polymer can be residues of benzimidazole-forming reagents, polybenzimidazoles are not mentioned in any of the above U.S. Pat. Nos. 4,378,400; 3,951,815; and 4,260,652.
For most separation processes using semipermeable membranes, two key parameters are the rejection value and flux. The rejection value is the relative ability of the membrane to retard the passage of the component or components being separated from the solution, e.g., a solute, usually expressed as a weight percentage of the total solute. Components rejected by semipermeable membranes include ionic materials such as NaCl and CaSO.sub.4 and organic materials such as sucrose or ethanol. Thus, the rejection value can also be described as a solute rejection rate. Flux refers to the amount of solvent passing through the membrane per unit area per unit time, generally expressed in gallons/square foot/day.
For the purposes of this application, a permselective membrane is one which is overall selectively permeable to at least the solvent in a solution, rejecting at least a portion of the solute contained therein.
Although semipermeable membranes and processes for their preparation are now developed to a high degree, improved processes to produce low cost membranes having relatively high flux and rejection values are desired. Permselective membranes for liquid reverse osmosis processes providing high flux and rejection values at low cost are particularly desired.
An object of this invention is to provide improved composite permselective membranes useful for liquid separations such as reverse osmosis, having improved properties of at least one of rejection value and flux as compared to conventional semipermeable membranes of polybenzimidazole polymers.
Another object of the invention is to provide processes for preparing such improved membranes. In another aspect, an object of the invention is to provide composite membranes offering performance characteristics comparable to those of semipermeable polybenzimidazole membranes, but at lower total cost.
A further object of the invention is composite permselective membranes which can be dried without becoming brittle.
Yet another object of the invention is composite permselective membranes which achieve high hydrolytic stability by incorporating a support membrane which is relatively insensitive to base or acid catalyzed hydrolysis.
These and other objects, as well as the scope, nature and utilization of the claimed invention will be apparent to those skilled in the art from the following detailed description, examples and the appended claims.